The present invention relates to well drilling operations and, in particular, to an apparatus for cutting and removing a core of a subterranean formation to be tested.
During the drilling of oil and gas wells it is desirable to periodically remove samples of the subterranean formation for analysis. The samples are obtained by means of a coring tool which is inserted into the well bore after the main drill bit has been raised from the bore. The coring tool includes a hollow bit which cuts a cylindrical core from the formation.
After the coring tool has cut the core, it is necessary for the core to be separated from the subterranean formation and raised to the surface. Traditionally, the coring tool has contained a mechanism which grips and, in effect, secures the core to the tool One such mechanism, disclosed for example in Knighton et al U.S. Pat. No. 4,606,416, contains a movable cam ring that is spring-biased downwardly by means of a compressed coil spring into engagement with pivotable gripping jaws. Subsequent to the core-cutting step, a jaw covering sleeve is raised to uncover the jaws, whereupon the spring actuated cam ring drives the jaws laterally into the core to penetrate and grip same.
Because available space within a coring tool is limited, the need to house a spring large enough to exert a sufficiently high force upon the jaws presents certain design problems. In that regard, it will be appreciated that the force exerted by a coil spring is a function of the length of its compression; thus, as the coil spring expands while driving the cam ring, the force which it exerts becomes progressively diminished. Hence, the size and strength of the spring must be such as to compensate for such behavior.
Furthermore, after the coring tool has been raised to the surface and stripped of its core it is necessary that the spring be recocked before the tool can be re-lowered to cut another sample core. That procedure adds to the time and difficulty involved in carrying out core sampling operations, involving the taking of multiple samples